I've had this 2001 Prius for a few weeks, the last week of which has been on a newly installed one-year-old (second-hand) battery, and the most I have yet to see the icon fill up, is to just to the bottom of the + Is this normal?
Yes it’s normal. The Prius leaves a buffer at the top in case you go down a hill. now if you have come down a long hill or a mountain and haven’t seen it full, then there’s a problem. but under normal driving, you won’t see it full. It might go up one level between where you are in the photo and full but never full.
I've seen mine go to the top once in the 18 years I've had one of these cars, just for a few minutes.